36 | New Scientist | 3 October 2020
To estimate the number of intelligent
civilisations capable of transmitting or
receiving radio signals within the Milky Way,
we often fall back on a formula drawn up by
astronomer Frank Drake in 1961. The Drake
equation multiplies seven variables, starting
with the rate of star formation in the galaxy,
the fraction of those stars with orbiting
planets and the fraction of those planets that
are habitable. Thanks largely to the Kepler
space telescope, which discovered thousands
of exoplanets before it retired in 2018, we
now know that pretty much all stars host
planets, many of which could harbour life.
That means we can use solid numbers for
several of the Drake equation’s terms.
But the calculation also contains other
biological variables. Here, we can do little
more than guess. What is the probability that,
given a habitable world, life gets started on it?
And if life does arise, what are the chances
that it becomes intelligent?
As things stand, these terms in the
Drake equation are so poorly known that
the calculation as a whole can end up spitting
out numbers that suggest we are alone in the
galaxy or instead that our civilisation is one
of millions. It all depends on what you put in.
A conventional approach to narrowing
down these probabilities would involve
doing some statistics. You observe a large
sample of Earth-like planets over billions
of years to see how frequently life arises and
how often any life that does emerge becomes
intelligent. The trouble is that we
have a sample size of one – Earth – and a
grand total of two data points concerning
it. We know that life appeared on our planet
fairly quickly after it was formed some
4.5 billion years ago – within the first 300
to 900 million years – while intelligence
is a much more recent development.
But there is another way to approach
probabilities, and it could change how
we think about the odds of finding alien
civilisations we could communicate with.
Bayesian statistics takes its name from
18th-century mathematician Thomas Bayes.
He came up with a way to calculate the
probability of a future event based on whatE
ARTH makes life look easy. Our home
planet is teeming with some 9 million
species, including at least one smart
enough to contemplate intelligent life
existing elsewhere in the universe. We love to
think it is out there – that we aren’t a one-off,
alone in an unimaginably vast cosmos.
A string of discoveries has boosted the idea
that extraterrestrial life might be abundant.
The growing catalogue of exoplanets, many
orbiting within the “habitable zone” of their
host stars, points to seemingly ample real
estate on which life might be found. Closer
to home, subsurface oceans on icy moons
in the outer solar system hint that some life-
friendly conditions might be commonplace.
And then there is last month’s discovery of
phosphine in the poisonous atmosphere of
Venus (see page 12), which suggests life might
flourish even in seemingly hostile places.
With all that in mind, it is easy to imagine
that intelligent life has evolved on at least
one planet around one of the 100 billion or
so stars in our galaxy. So easy, in fact, that wetend to assume, given the vastness of the
visible universe, that there must be other
technological civilisations out there.
Yet we haven’t heard from them. Why?
In the absence of evidence from deep
space, some astronomers have recently
returned their focus to Earth – and the
only example of intelligent life we have –
for a fresh look at the question. What they
found meshes with what biologists have
been whispering for a while: that anyone
expecting to hear from an alien civilisation
should settle in for a long wait.A famous formula
For all its lack of success, the search for
extraterrestrial intelligence (SETI) has
never lacked for optimism. For decades,
SETI researchers have swept the skies with
radio telescopes in the hope of finding
messages from another technological
civilisation. But the truth is that we have
no idea if there is anybody out there. >Features Cover story
Is anybody
out there?
News from Venus has raised fresh hopes of finding
alien life right next door – but that is the easy bit.
What are the chances we will find intelligent life
elsewhere in the universe, asks Dan Falk